Flattening the Globe

Every map is untrue in a particular way. A familiar way to see the problem is to try to flatten the peel of an orange without tearing it. The Earth is very nearly a sphere, and a sphere cannot be flattened onto paper without stretching, tearing or squeezing some part of its surface. Mapmakers have known this for centuries, and the methods they use to turn a globe into a page, called projections, are best understood as a series of compromises about which distortions to accept. The most famous compromise was published in 1569 by a Flemish geographer, Gerardus Mercator, for sailors. On his map, lines of constant compass bearing appear as straight lines, which means a navigator can plot a course with a ruler and follow it by keeping a fixed heading. To achieve this, Mercator had to stretch the map progressively towards the poles. Greenland therefore appears roughly as large as Africa, although Africa is about fourteen times bigger. The map was an immediate success among navigators, and variations of it are still printed on ships' charts today. For navigation the stretching was a price worth paying, but when the projection moved from ships' cabins to school walls, its distortions came to shape how people imagine the world. Critics say that it exaggerates the importance of Europe and North America, which lie in the stretched northern band, and diminishes the tropical regions where many people live. Several alternative maps have been proposed in response. Some preserve the relative size of each country but bend the shapes of the continents, while others keep shapes recognisable near the centre and let the edges curve away. Another design divides the globe into separate pieces, like the segments of an orange peel, so that distortion within each piece is small but the oceans are cut apart. No alternative, however, is free of cost. A map that shows area correctly cannot also show every angle correctly; this is a mathematical limit, not a failure of ingenuity. Cartographers therefore choose according to purpose. A map for comparing population density needs equal areas, one for flight planning needs routes that follow great circles, and one for a ship's bridge still, in practice, needs Mercator's straight lines. Digital maps have revived the old debate. Most online mapping services display a version of Mercator's projection, chosen because it keeps the shapes of streets accurate at every level of zoom, which matters when someone is following a route on a phone. The result is that more people than ever view the world through a sixteenth century solution. In my view, the answer is not to ban the projection but to teach people to read it: a student who knows why Greenland looks enormous is better equipped than one who has merely been shown a different picture.